Pinnipeds (from Latin pinna, wing or fin, and ped-, foot) or fin-footed mammals are a widely distributed and diverse group of semiaquatic marine mammals comprising the families Odobenidae (the walrus), Otariidae (eared seals, including sea lions and fur seals), and Phocidae (earless seals).

Pinnipeds are typically sleek-bodied and barrel-shaped. Their bodies are well adapted to the aquatic habitat where they spend most of their lives. Their limbs consist of short, wide, flat flippers. The smallest pinniped, the Baikal seal, weighs about 70 kg (155 lb) on average when full-grown and is 1.3 m (4 ft 3 in) long; the largest, the male southern elephant seal, is over 4 metres (13 ft) long and weighs up to 4,000 kilograms (8,800 lb).

Earless seals, also called true seals, or phocids, are the most diverse and widespread pinnipeds. They lack external ears, have more streamlined snouts, and are generally more aquatically adapted than otariids. They swim with efficient, undulating whole-body movements using their more-developed rear flippers. The swimming efficiency and an array of other physiological adaptations make them better built for deep and long diving and long distance migration. These mammals are, however, very clumsy on land, moving by wriggling their front flippers and abdominal muscles. The two back flippers form a tail-like structure which does not aid walking on land. True seals generally communicate by slapping the water and grunting rather than vocalising.